1. Field of the Invention
This invention relates to a heat treating furnace, and more particularly to a heat treating furnace capable of performing heat treatments under high pressure. The heat treating furnace provides a double-chamber structure including a gas circulation chamber and a reaction chamber. By controlling the relative gas density and pressure of the chambers, the reaction gases can be mixed uniformly and the reaction could be facilitated under high pressure. Hence the quality of the formed thin film and the operational safety are improved.
2. Description of the Prior Art
With the development of compound thin film solar cell technologies, the thin film fabrication have been used in generating more and more products, thus the demand of equipments for developing the thin film or the thin film precursor on substrates is greatly increased. However, the present methods of developing the thin film include spattering and co-evaporation. Especially for fabricating the products which are mass produced successfully in thin-film photovoltaic industry, spattering is the most commonly used technique in developing the thin film precursor prior to the chemical reaction process to form the thin film.
Furthermore, among the techniques of performing chemical reaction processes on the thin film precursor for forming thin films, providing chemical compound vapor is the most suitable method for mass production. It is an advantageous way of providing chemical compound vapor to supply the required elements for forming the thin film precursor, such that the concentration and the diffusion of ingredients for forming the thin film precursor can be accurately controlled. As a result, the development of techniques and equipments of performing chemical reactions for forming thin films which employ the heat treating furnace grows vigorously. Taking the selenization process of Copper Indium Gallium Diselenide (CIGS) solar cell as an example, the spattering deposition technique is used for forming multiple-layer precursors containing alloys or monomers of copper (Cu), gallium (Ga) and indium (In) on a soda lime glass substrate to constitute the structure of CIGS solar cell. Then the layered structure for producing CIGS solar cell is transferred into a selenization furnace (i.e. heat treating furnace), and the gaseous hydrogen selenide (H2Se) is introduced into the selenization furnace and is heated to the temperature of 400° C. or a higher temperature to start the reaction between the gaseous hydrogen selenide and the multiple-layer precursors. However, the selenization process of CIGS solar cell fabrication, heating the solar cell structure with multiple-layer thin films is required for reacting with gaseous hydrogen selenide to produce the high-quality CIGS films. For example, a copper-gallium (Cu—Ga) alloy layer, a copper-indium (Cu—In) alloy layer and an indium layer are deposited to form the three-layer precursor (CuGa/CuIn/In) film of uniform thickness. The three-layer precursor film is transferred into a selenization furnace immediately after the deposition. Then the gaseous hydrogen selenide is introduced and the three-layer precursor film is heated to the temperature of 400° C. at the heating rate of 40° C./min, and the three-layer precursor film is reacted with selenide to form a compound CIGS layer. The compound CIGS layer is then heated to 550° C. at the heating rate of 15° C./min to provide the optimal crystal structure, followed by a step of cooling, and the compound CIGS layer is formed.
Due to that the selenization process is performed at the temperature range of 520 to 590° C., a large thick quartz tubes is utilized to be the inner body in the conventional heat treating furnace, and the outer side is tightly contacted to the thermal insulating materials, as a result, inside the heat treating furnace is in a closed status. In addition, the effects of thermal expansion and contraction makes the reaction gas with higher temperature flowing upward and the reaction gas with lower temperature flowing downward, which result in poor gas mixing in the selenization process, thus further result in variant quality and the thickness of the compound CIGS layer on the glass substrate. Furthermore, the reaction gases such as hydrogen selenide used in the selenization process are toxic; therefore the pressure inside the selenization furnace needs to be controlled at low pressure (i.e. lower than 1 atm) throughout the whole selenization process for the safety considerations and avoids the leakage of reaction gases otherwise causes industrial safety concerns. In this situation, the selenization process under low pressure evokes insufficient total gas molecules and results in the deterioration of the temperature gradient inside the selenization furnace, and also deteriorates the gas mixing uniformity. Those events result in a vicious circle that slow down the reaction rate and simultaneously worsen the uniformity of thin film. Apparently, the low pressure and the non-uniform temperature of present selenization furnaces generally result in the problems of selenium gas heterogeneity and ineffective thin film formation, thus the ultimate difficulty of promoting the photovoltaic conversion efficiency.
FIG. 1a and 1b show schematically a prior art the embodiment of U.S. Pat. No. 7,871,502 patent. Referring to FIG. 1a, the selenization furnace includes only one closed reaction chamber provided for the selenization process of compound CIGS, and the pressure inside the chamber is kept lower than 1 atm throughout the whole selenization process. FIG. 1b shows the temperature profile diagram of selenization process. FIG. 1c shows the temperature and the pressure profile of the selenization process inside the selenization furnace shown in FIG. 1a. After closing the selenization furnaces, repeatedly pumping out the air inside the selenization furnace and pumping gaseous nitrogen into the reaction chamber is required to ensure that the reaction chamber is full of gaseous nitrogen. The operational pressure of reaction chamber of the conventional selenization furnace is kept at low pressure (i.e. lower than 1 atm) concerning the safety. The pressure in the reaction camber is controlled within a range of 0.8 to 0.9 atm throughout the whole reaction process. The gaseous pressure inside the reaction chamber increases as the selenization furnace is heated to the temperature of 590° C., thus, the gas is eliminated repeatedly for the purpose of reducing the pressure to maintain the pressure inside the reaction chamber at a set point. However, during the process of gas elimination, energy and excessive gas are wasted. When the temperature reaches the set point for reaction, the reaction gases are introduced simultaneously into the reaction chamber. Generally, hydrogen selenide (10%) and gaseous nitrogen (90%) which is the carrier gas are used for reaction. As shown in FIG. 1c, the reaction time of selenization is less than 100 minutes, but apparently the gas flows in the reaction chamber cannot be convected and the temperature cannot be uniformed in such short reaction time. Therefore, the uniformity of selenization is deteriorated which causes the variant thickness and quality of the compound CIGS layer on the substrates.
Following the reaction of forming the compound CIGS layer, the selenization furnace needs to be cooled down to transfer the CIGS solar cell substrate out of the selenization furnace. However, the reaction chamber of the inner body is a closed space, the only way to cool down the selenization furnace is pumping the gaseous nitrogen into inner body of the furnace and pumping out the gas at the same time which is a time consuming cooling process. As shown in FIG. 1c, this cooling process generally takes 5 to 8 hours, but when it comes to a larger substrate, it takes even more than 10 hours. Thus, tremendous manpower and resources are required, resulting in retarding the fabrication. Additionally, as shown in FIG. 1a, the gas pipes and the signal transmission circuit of the selenization furnace are located on the gate doors. However the gate doors need to be frequently opened in the fabrication, which may result in loosening or fracturing the gas pipes and the signal transmission circuit, making the operation being hazardous. In view of this, attempts have been made in the present invention to solve the aforementioned problems and drawbacks by providing a newly improved heat treating furnace.